Enantioselective Carbonyl Catalysis Enabled by Chiral Aldehydes

Radical meta-C-H Halogenation of Azines via N-Benzyl Activation Strategy

Regioselective halogenation of six-membered N-heteroarenes is crucial for precise functional derivatization. We present a meta-selective halogenation method for pyridines, quinolines, and isoquinolines via electrophilic halogen radical addition utilizing an N-benzyl activation strategy. This method achieves C3- and C5-dihalogenation in pyridines, C3- and C6-dihalogenation in quinolines, and C3-monohalogenation in isoquinolines. The feasibility and potential applications of this method were validated through scale-up reactions and the bromination of quinoline derivatives with biomolecular fragments.

Asymmetric three-component Tsuji-Trost allylation reaction enabled by chiral aldehyde/palladium combined catalysis

Despite the long-standing exploration of the catalytic asymmetric Tsuji-Trost allylation reaction since the mid-20th century, most reported instances have adhered to a two-component approach. Here, we present a remarkably efficient three-component asymmetric allylation reaction enabled by the collaborative action of chiral aldehyde and palladium. A diverse array of NH2-unprotected amino acid esters, aryl or alkenyl iodides, and allyl alcohol esters exhibit robust participation in this reaction, resulting in the synthesis of structurally diverse non-proteinogenic α-amino acid esters with favorable experimental outcomes. Mechanistic investigations reveal the dominance of the allylation/Heck coupling cascade in reactions involving electron-rich aryl iodides, while the Heck coupling/allylation cascade emerges as the dominant pathway in reactions involving electron-deficient aryl iodides. This chiral aldehyde/palladium combining catalytic system precisely governs the chemoselectivity of C-allylation and N-allylation, the regioselectivity of linear and branched allylation, and the enantioselectivity of C-allylation products.

A New Age of Biocatalysis Enabled by Generic Activation Modes

Biocatalysis is currently undergoing a profound transformation. The field moves from relying on nature's chemical logic to a discipline that exploits generic activation modes, allowing for novel biocatalytic reactions and, in many instances, entirely new chemistry. Generic activation modes enable a wide range of reaction types and played a pivotal role in advancing the fields of organo- and photocatalysis. This perspective aims to summarize the principal activation modes harnessed in enzymes to develop new biocatalysts. Although extensively researched in the past, the highlighted activation modes, when applied within enzyme active sites, facilitate chemical transformations that have largely eluded efficient and selective catalysis. This advance is attributed to multiple tunable interactions in the substrate binding pocket that precisely control competing reaction pathways and transition states. We will highlight cases of new synthetic methodologies achieved by engineered enzymes and will provide insights into potential future developments in this rapidly evolving field.

Stereoselective amino acid synthesis by photobiocatalytic oxidative coupling

Photobiocatalysis-where light is used to expand the reactivity of an enzyme-has recently emerged as a powerful strategy to develop chemistries that are new to nature. These systems have shown potential in asymmetric radical reactions that have long eluded small-molecule catalysts1. So far, unnatural photobiocatalytic reactions are limited to overall reductive and redox-neutral processes2-9. Here we report photobiocatalytic asymmetric sp3-sp3 oxidative cross-coupling between organoboron reagents and amino acids. This reaction requires the cooperative use of engineered pyridoxal biocatalysts, photoredox catalysts and an oxidizing agent. We repurpose a family of pyridoxal-5'-phosphate-dependent enzymes, threonine aldolases10-12, for the α-C-H functionalization of glycine and α-branched amino acid substrates by a radical mechanism, giving rise to a range of α-tri- and tetrasubstituted non-canonical amino acids 13-15 possessing up to two contiguous stereocentres. Directed evolution of pyridoxal radical enzymes allowed primary and secondary radical precursors, including benzyl, allyl and alkylboron reagents, to be coupled in an enantio- and diastereocontrolled fashion. Cooperative photoredox-pyridoxal biocatalysis provides a platform for sp3-sp3 oxidative coupling16, permitting the stereoselective, intermolecular free-radical transformations that are unknown to chemistry or biology.

Chiral Aldehyde Catalysis-Enabled Asymmetric α-Functionalization of Activated Primary Amines

ConspectusThe development of catalytic activation modes provides a reliable and effective platform for designing new enantioselective reactions and preparing chiral molecules with diverse structures. Chiral aldehyde catalysis is an attractive concept in asymmetric catalysis, which utilizes a chiral aldehyde catalyst to promote the asymmetric hydroamination of allylic amines, the asymmetric α-functionalization of primary amines, or the asymmetric transamination of α-keto esters. Typically, the chiral aldehyde-catalyzed asymmetric α-functionalization of primary amines provides an efficient and straightforward method for the synthesis of α-functionalized chiral amines, which does not require any additional protection or deprotection manipulations of the amine group. However, achieving catalytic stereoselective transformations with high efficiency and enantioselectivity by this strategy has remained an intractable challenge.This Account summarizes our endeavors in the development and application of chiral aldehyde catalysis. Using a chiral aldehyde as a catalyst, we reported the catalytic asymmetric α-C alkylation of 2-aminomalonate with 3-indolylmethanol in 2014, which represents the first chiral aldehyde-catalyzed asymmetric α-functionalization of an activated primary amine. Subsequently, several axially chiral aldehyde catalysts were continuously prepared by using chiral BINOL as the starting material, and their applications in asymmetric synthesis were explored. On the one hand, they were used as organocatalysts to realize the various transformations of α-amino acid esters, such as asymmetric 1,4-addition toward conjugated enones/α,β-unsaturated diesters and cyclic 1-azadienes as well as asymmetric α-arylation/allylation and benzylation with corresponding halohydrocarbons. Notably, taking advantage of the difference in the distribution of catalytic sites between two chiral aldehyde catalysts, we disclosed chiral aldehyde-catalyzed diastereodivergent 1,6-conjugated addition and Mannich reactions. On the other hand, the potential for the cooperative catalysis of a chiral aldehyde with a transition metal has also been demonstrated. Enabled by the combination of a chiral aldehyde, a palladium complex, and a Lewis acid, the enantioselective α-allylation of amino acid esters with allyl alcohol esters was established. Moreover, the ternary catalytic system has been successfully used for the α-functionalization of amino acid esters with 1,3-dienes, allenes, allenylic alcohol esters, 1,3-disubstituted allyl alcohol esters, and arylmethanol esters as well as the asymmetric cascade Heck-alkylation reaction. The combination of a chiral aldehyde and nickel complex allows for the asymmetric α-propargylation of amino acid esters with propargylic alcohol esters and provides excellent enantioselectivities. These transformations provide a large library of optically active amines and amino acids. With those chiral amino acid esters as key building blocks, the synthesis or formal synthesis of multiple natural products and biologically significant unnatural molecules was accomplished. This includes the stereodivergent synthesis of natural pyrrolizidine alkaloid NP25302 and the formal synthesis of natural product (S)-hypoestestatin 1 and manzacidin C, clinical candidate compound (+)-AG-041R, and somatostatin mimetics. It is fully anticipated that chiral aldehyde catalysis will soon witness rapid expansion both in the development of novel asymmetric transformations and in innovative applications for constructing optically active nitrogen-containing molecules with significant values.

Direct Transamidation of Thioamides with Amines via Acetophenone-Promoted Enamine Catalysis under Metal-Free Conditions

Herein, we developed a highly selective, efficient, and simple method for direct transamidation of thioamides with amines, promoted by commercially available acetophenone under metal-/solvent-free conditions. The reaction tolerated a wide range of functional groups and substrates, including single- or double-thioamides, benzylamines, or alkyl/cycloalkyl-substituted aliphatic amines. The present protocol can be applied to gram-scale in good yields. In addition, the Pt-/Ni-complexes of double-transamidation products were obtained in good to excellent yields. The investigation of photophysical properties indicated that the fluorescence spectra of Pt-complexes showed an emission band centered at 550-750 nm and exhibited red fluorescence when irradiated by a UV lamp (365 nm).

Asymmetric α-C(sp3)-H allylic alkylation of primary alkylamines by synergistic Ir/ketone catalysis

Primary alkyl amines are highly reactive in N-nucleophilic reactions with electrophiles. However, their α-C-H bonds are unreactive towards electrophiles due to their extremely low acidity (pKa ~57). Nonetheless, 1,8-diazafluoren-9-one (DFO) can activate primary alkyl amines by increasing the acidity of the α-amino C-H bonds by up to 1044 times. This makes the α-amino C-H bonds acidic enough to be deprotonated under mild conditions. By combining DFO with an iridium catalyst, direct asymmetric α-C-H alkylation of NH2-unprotected primary alkyl amines with allylic carbonates has been achieved. This reaction produces a wide range of chiral homoallylic amines with high enantiopurities. The approach has successfully switched the reactivity between primary alkyl amines and allylic carbonates from intrinsic allylic amination to the α-C-H alkylation, enabling the construction of pharmaceutically significant chiral homoallylic amines from readily available primary alkyl amines in a single step.

Access to Axially Chiral Aryl Aldehydes via Carbene-Catalyzed Nitrile Formation and Desymmetrization Reaction

An approach utilizing N-heterocyclic carbene for nitrile formation and desymmetrization reaction is developed. The process involves kinetic resolution, with the axially chiral aryl monoaldehydes obtained in moderate yields with excellent optical purities. These axially chiral aryl monoaldehydes can be conveniently transformed into functionalized molecules, showing great potential as catalysts in organic chemistry.

Core Structure-Oriented Asymmetric α-Allenylic Alkylation of Amino Acid Esters Enabled by Chiral Aldehyde/Palladium Catalysis

Aiming at the reported chiral synthons leading to manzacidins A and D, here we report a highly efficient catalytic asymmetric α-allenylic alkylation reaction of NH2-unprotected amino acid esters that is promoted by combined chiral aldehyde/palladium catalysis. Fifty examples of unnatural α,α-disubstituted amino acid esters are reported with good-to-excellent yields and stereoselectivities. Based on this methodology, a key intermediate leading to manzacidin C and its other three stereoisomers is prepared accordingly.

Chiral Carbon Dots and Chiral Carbon Dots with Circularly Polarized Luminescence: Synthesis, Mechanistic Investigation and Applications

Chiral carbon dots (CCDs) can be widely used in various fields such as chiral recognition, chiral catalysis and biomedicine because of their unique optical properties, low toxicity and good biocompatibility. In addition, CCDs with circularly polarized luminescence (CPL) can be synthesized, thus broadening the prospects of CCDs applications. Since the research on CCDs is still in its infancy, this paper reviews the chiral origin, formation mechanism, chiral evolution, synthesis and emerging applications of CCDs, with a special focus on CCDs with CPL activity. It is hoped that it will provide some reference to solve the current problems faced by CCDs. Finally, the opportunities and challenges of the current research on CCDs are described, and their future development trends have also been prospected.

The Development of Aldehyde Catalytic System

Aldehyde catalysts have proven to be highly effective in facilitating and accelerating a wide range of challenging transformations in organic chemistry. This article is structured into three main sections, focusing on the utilization of aldehydes as organocatalysts, the aldehydes/transition metals catalytic systems, and photochemical initiators. Finally, we provide a concise summary of the advancements in this fascinating research field, offering our perspectives and insights.

Iridium(I)-Catalyzed Atroposelective Alkenylation of Heterobiaryls with Terminal Alkynes

Herein we report an iridium(I)-catalyzed atroposelective alkenylation of isoquinoline-derived heterobiaryls with terminal alkynes. In the presence of a cationic iridium(I) catalyst with (R)-SEGPHOS as the chiral ligand, this atom-economical alkenylation protocol allows the rapid construction of a series of axially chiral alkenylated heterobiaryls in moderate to good yields with good to high enantioselectivities.

Picolinaldehyde-Zinc(II)-Palladium(0) Catalytic System for the Asymmetric α-Allylation of N-Unprotected Amino Esters

Reported in this work is a synergistic ternary achiral picolinaldehyde-Zn(II)-chiral palladium complex system for the highly enantioselective α-allylation of N-unprotected amino esters. By utilizing a variety of allylic carbonates or vinyl benzoxazinanones as substrates, α-allyl α-amino esters were obtained in high yields (up to 96 %) with high enantioselectivities (up to 98 % ee). Control experiments suggest that the coordination of Zn(II) with the Schiff base intermediate enhances the acidity of the α-C-H bonds of amino esters, thereby favoring α-allylation over intrinsic N-allylation. Furthermore, NMR studies reveal an interaction between the chiral palladium complex and the Zn(II)-Schiff base intermediate, leading to the formation of a picolinaldehyde-Zn(II)-Pd(0) catalytic system.

Stereoselective amino acid synthesis by synergistic photoredox-pyridoxal radical biocatalysis

Developing synthetically useful enzymatic reactions that are not known in biochemistry and organic chemistry is an important challenge in biocatalysis. Through the synergistic merger of photoredox catalysis and pyridoxal 5'-phosphate (PLP) biocatalysis, we developed a pyridoxal radical biocatalysis approach to prepare valuable noncanonical amino acids, including those bearing a stereochemical dyad or triad, without the need for protecting groups. Using engineered PLP enzymes, either enantiomeric product could be produced in a biocatalyst-controlled fashion. Synergistic photoredox-pyridoxal radical biocatalysis represents a powerful platform with which to discover previously unknown catalytic reactions and to tame radical intermediates for asymmetric catalysis.

Chiral aldehyde catalysis enables direct asymmetric α-substitution reaction of N-unprotected amino acids with halohydrocarbons

The direct catalytic α-hydrocarbylation of readily available amino acids with halohydrocarbons is one of the most straightforward methods leading to α,α-disubstituted non-proteinogenic α-amino acid compounds. However, all the reported methodologies depend on N-protected amino acids as starting materials. Herein, we report on three highly efficient aldehyde-catalyzed direct α-hydrocarbylations of N-unprotected amino acid esters with aryl-, allyl-, and benzyl halides. By promoting a simple chiral BINOL-aldehyde catalyst or combining catalysts of a chiral aldehyde and Lewis acid ZnCl₂, the asymmetric α-arylation, α-allylation, and α-benzylation of amino acid esters with the corresponding halohydrocarbons proceed smoothly, producing α,α-disubstituted α-amino acids in moderate-to-high yields and good-to-excellent enantioselectivities. The asymmetric α-arylation reaction can be applied in the formal synthesis of the clinical candidate compound (+)-AG-041R. Based on the results given by control experiments, three reaction models are proposed to illustrate the stereoselective-control outcomes.

Vitamin B6-Based Biomimetic Asymmetric Catalysis

ConspectusOne of the fundamental goals of chemists is to develop highly efficient methods for producing optically active compounds, given their wide range of applications in chemistry, pharmaceutical industry, chemical biology, and material science. Biomimetic asymmetric catalysis, which imitates the structures and functions of enzymes, has emerged as an extremely attractive strategy for producing chiral compounds. This field has drawn tremendous research interest and has led to various protocols for constructing complex molecular scaffolds. The Vitamin B₆ family, including pyridoxal, pyridoxamine, pyridoxine, and the corresponding phosphorylated derivatives, serves as the cofactors to catalyze more than 200 enzymatic functions, accounting for ∼4% of all enzyme activities. Although significant progress has been made in simulating the biological roles of vitamin B₆ during the past several decades, its extraordinary catalytic power has not yet been successfully applied into asymmetric synthesis. In recent years, our group has been devoted to developing vitamin B₆-based biomimetic asymmetric catalysis using chiral pyridoxals/pyridoxamines as catalysts. We are particularly interested in mimicking the processes of enzymatic transamination and biological aldol reaction of glycine, respectively, developing asymmetric biomimetic transamination and carbonyl catalysis enabled α-C-H transformation of primary amines. Using a chiral α,α-diarylprolinol-derived pyridoxal as the catalyst, we reported the first chiral pyridoxal catalyzed asymmetric transamination of α-keto acids in 2015. A significant breakthrough in biomimetic transamination was achieved by using an axially chiral biaryl pyridoxamine catalyst that bears a lateral amine side arm. The amine side arm acts as an intramolecular base, accelerating the transamination and proving highly effective for transamination of α-keto acids and α-keto amides. In addition, we discovered the catalytic power of chiral pyridoxals as carbonyl catalysts for asymmetric biomimetic Mannich/aldol reactions of glycinates. These chiral pyridoxals also enabled more α-C-H conversions of glycinates, such as asymmetric 1,4-addition toward α,β-unsaturated esters and asymmetric α-allylation with Morita-Baylis-Hillman acetates. Moreover, carbonyl catalysis can be further applied to highly challenging primary amines with inert α-C-H bonds, such as propargylamines and benzylamines, which represents a powerful strategy for direct asymmetric α-C-H functionalization of various primary amines without protection of the NH₂ group. These biomimetic/bioinspired transformations provide efficient new protocols for the synthesis of chiral amines. Herein, we summarize our recent efforts on the development of the vitamin B₆-based biomimetic asymmetric catalysis.

Biomimetic Catalytic Retro-Aldol Reaction Using a Cation-Binding Catalyst: A Promising Route to Axially Chiral Biaryl Aldehydes

Here we describe a biomimetic catalytic retro-aldol reaction of racemic α-substituted β-hydroxy ketones utilizing a chiral oligoEG cation-binding catalyst as a type-II aldolase mimic. Our investigation of various aldol substrates has demonstrated that our biomimetic retro-aldol protocol enables rapid access to highly enantiomerically enriched aldols with a selectivity factor (s) of up to 70. Additionally, we have demonstrated the synthetic strategy's feasibility for accessing diverse and valuable axially chiral aldehydes.

Diastereo- and enantioselective synthesis of biaryl aldehydes bearing both axial and central chirality

An unprecedented method for the synthesis of biaryl aldehydes bearing both axial and central chirality is presented.

Chiral aldehyde-nickel dual catalysis enables asymmetric α-propargylation of amino acids and stereodivergent synthesis of NP25302

The combined catalytic systems derived from organocatalysts and transition metals exhibit powerful activation and stereoselective-control abilities in asymmetric catalysis. This work describes a highly efficient chiral aldehyde-nickel dual catalytic system and its application for the direct asymmetric α-propargylation reaction of amino acid esters with propargylic alcohol derivatives. Various structural diversity α,α-disubstituted non-proteinogenic α-amino acid esters are produced in good-to-excellent yields and enantioselectivities. Furthermore, a stereodivergent synthesis of natural product NP25302 is achieved, and a reasonable reaction mechanism is proposed to illustrate the observed stereoselectivity based on the results of control experiments, nonlinear effect investigation, and HRMS detection.

Ruthenium(II)/Imidazolidine Carboxylic Acid-Catalyzed C-H Alkylation for Central and Axial Double Enantio-Induction

Enantioselective C-H activation has surfaced as a transformative toolbox for the efficient assembly of chiral molecules. However, despite of major advances in rhodium and palladium catalysis, ruthenium(II)-catalyzed enantioselective C-H activation has thus far largely proven elusive. In contrast, we herein report on a ruthenium(II)-catalyzed highly regio-, diastereo- and enantioselective C-H alkylation. The key to success was represented by the identification of novel C2-symmetric chiral imidazolidine carboxylic acids (CICAs), which are easily accessible in a one-pot fashion, as highly effective chiral ligands. This ruthenium/CICA system enabled the efficient installation of central and axial chirality, and featured excellent branched to linear ratios with generally >20 : 1 dr and up to 98 : 2 er. Mechanistic studies by experiment and computation were carried out to understand the catalyst mode of action.

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

Catalytic asymmetric Tsuji-Trost α-benzylation reaction of N-unprotected amino acids and benzyl alcohol derivatives

Catalytic asymmetric Tsuji–Trost benzylation is a promising strategy for the preparation of chiral benzylic compounds. However, only a few such transformations with both good yields and enantioselectivities have been achieved since this reaction was first reported in 1992, and its use in current organic synthesis is restricted. In this work, we use N-unprotected amino acid esters as nucleophiles in reactions with benzyl alcohol derivatives. A ternary catalyst comprising a chiral aldehyde, a palladium species, and a Lewis acid is used to promote the reaction. Both mono- and polycyclic benzyl alcohols are excellent benzylation reagents. Various unnatural optically active α-benzyl amino acids are produced in good-to-excellent yields and with good-to-excellent enantioselectivities. This catalytic asymmetric method is used for the formal synthesis of two somatostatin mimetics and the proposed structure of natural product hypoestestatin 1. A mechanism that plausibly explains the stereoselective control is proposed.

The catalytic asymmetric benzylations of prochiral nucleophiles are very limited. Here, the authors disclose an asymmetric α−benzylation of N-unprotected amino acids with benzyl alcohol derivatives by a chiral aldehyde-involved catalytic system.

Asymmetric α-Allylation of Glycinate with Switched Chemoselectivity Enabled by Customized Bifunctional Pyridoxal Catalysts

Owing to the strong nucleophilicity of the NH₂ group, free-NH₂ glycinates react with MBH acetates to usually deliver N-allylated products even in the absence of catalysts. Without protection of the NH₂ group, chiral pyridoxal catalysts bearing an amide side chain at the C3 position of the naphthyl ring switched the chemoselectivity of the glycinates from intrinsic N-allylation to α-C allylation. The reaction formed chiral multisubstituted glutamic acid esters as S_N 2'-S_N 2' products in good yields with excellent stereoselectivity (up to 86 % yield, >20 : 1 dr, 97 % ee). As compared to pyridoxal catalysts bearing an amide side arm at the C2 position, the pyridoxals in this study have a bigger catalytic cavity to enable effective activation of larger electrophiles, such as MBH acetates and related intermediates. The reaction is proposed to proceed via a cooperative bifunctional catalysis pathway, which accounts for the high level of diastereo- and enantiocontrol of the pyridoxal catalysts.

Synthesis of Axially Chiral Aldehydes by N-Heterocyclic-Carbene-Catalyzed Desymmetrization Followed by Kinetic Resolution

Axially chiral aldehydes have received increasing attention in enantioselective catalysis. However, only very few catalytic methods have been developed to construct structurally diverse axially chiral aldehydes. We herein describe an NHC-catalyzed atroposelective esterification of biaryl dialdehydes as a general and practical strategy for the construction of axially chiral aldehydes. Mechanistic studies indicate that coupling proceeds through a novel combination of NHC-catalyzed desymmetrization of the dialdehydes and kinetic resolution. This protocol features excellent enantioselectivity, mild conditions, good functional-group tolerance, and applicability to late-stage functionalization and provides a modular platform for the synthesis of axially chiral aldehydes and their derivatives.

Thioether-enabled palladium-catalyzed atroposelective C–H olefination for N−C and C−C axial chirality

Thioethers allowed for highly atroposelective C–H olefinations by a palladium/chiral phosphoric acid catalytic system under ambient air. Both N–C and C–C axial chiral (hetero)biaryls were successfully constructed, leading to a broad range of axially chiral N-aryl indoles and biaryls with excellent enantioselectivities up to 99% ee. Experimental and computational studies were conducted to unravel the walking mode for the atroposelective C–H olefination. A plausible chiral induction model for the enantioselectivity-determining step was established by detailed DFT calculations.

Thioethers allowed for highly atroposelective C–H olefinations by a palladium/chiral phosphoric acid catalytic system under ambient air.

Chiral aldehyde induced tandem conjugated addition-lactamization reaction for constructing full-substituted pyroglutamic acids

A catalytic asymmetric tandem reaction including a chiral aldehyde catalyzed conjugated addition and an intramolecular lactamization is reported in this work. Under the optimal reaction conditions, various full-substituted pyroglutamic acids...

Iridium(iii)-catalyzed two-fold C–H alkylation of BINOLs with allyl alcohols

Ir(iii)-Catalyzed C–H alkylation of BINOL units has been well examined by using allyl alcohols as coupling partners.

Organocatalytic atroposelective heterocycloaddition to access axially chiral 2-arylquinolines

Axially chiral heterobiaryls play a vital role in asymmetric synthesis and drug discovery. However, there are few reports on the synthesis of atropisomeric heterobiaryls compared with axially chiral biaryls. Thus, the rapid enantioselective construction of optically active heterobiaryls and their analogues remains an attractive challenge. Here, we report a concise chiral amine-catalyzed atroposelective heterocycloaddition reaction of alkynes with ortho-aminoarylaldehydes, and obtain a new class of axially chiral 2-arylquinoline skeletons with high yields and excellent enantioselectivities. In addition, the axially chiral 2-arylquinoline framework with different substituents is expected to be widely used in enantioselective synthesis.

Efficient Asymmetric Biomimetic Aldol Reaction of Glycinates and Trifluoromethyl Ketones by Carbonyl Catalysis

The direct asymmetric aldol reaction of glycinates represents an intriguing and straightforward strategy to make biologically significant chiral β-hydroxy-α-amino-acid derivatives. But it is not easy to realize the transformation due to the disruption of the reactive NH₂ group of glycinates. Inspired by the enzymatic aldol reaction of glycine, we successfully developed an asymmetric aldol reaction of glycinate 5 and trifluoromethyl ketones 4 with 0.1-0.0033 mol % of chiral N-methyl pyridoxal 7 a as the catalyst, producing chiral β-trifluoromethyl-β-hydroxy-α-amino-acid esters 6 in 55-82 % yields (for the syn-diastereomers) with up to >20:1 dr and 99 % ee under very mild conditions. The reaction proceeds via a catalytic cycle similar to the enzymatic aldol reaction of glycine. Pyridoxal catalyst 7 a activates both reactants at the same time and brings them together in a specific spatial orientation, accounting for the high efficiency as well as excellent diastereo- and enantioselectivities.

Aldehyde-catalyzed epoxidation of unactivated alkenes with aqueous hydrogen peroxide

The organocatalytic epoxidation of unactivated alkenes using aqueous hydrogen peroxide provides various indispensable products and intermediates in a sustainable manner. While formyl functionalities typically undergo irreversible oxidations when activating an oxidant, an atropisomeric two-axis aldehyde capable of catalytic turnover was identified for high-yielding epoxidations of cyclic and acyclic alkenes. The relative configuration of the stereogenic axes of the catalyst and the resulting proximity of the aldehyde and backbone residues resulted in high catalytic efficiencies. Mechanistic studies support a non-radical alkene oxidation by an aldehyde-derived dioxirane intermediate generated from hydrogen peroxide through the Payne and Criegee intermediates.

Catalytic Asymmetric Amino Acid and Its Derivatives by Chiral Aldehyde Catalysis

Amine acid transformation is an important chemical process in biological systems. As a well-developed and acknowledged tool, chiral aldehyde catalysis provides good catalytic activation and stereoselective control abilities in the asymmetric reaction of N-unprotected amino acid esters and amino acid esters analogs, in which the key to success is the design of the catalysts derived from chiral BINOL aldehyde, which is based on the face control of enolate intermediates. In this review, one of the co-catalytic systems that combined with a transition metal to form a multiplex catalytic system and the well-established multiplex stereocenters of chiral aldehyde catalysis have been reviewed. Finally, a novel organocatalysis is prospected.

Mechanistic Study of the N-Quaternized Pyridoxal-Catalyzed Biomimetic Asymmetric Mannich Reaction: Insights into the Origins of Enantioselectivity and Diastereoselectivity

Density functional theory calculations have been performed to gain insights into the catalytic mechanism of the N-quaternized pyridoxal (i.e., 1a)-mediated biomimetic asymmetric Mannich reaction of tert-butyl glycinate 3 with N-diphenylphosphinyl imine 2a to give the diamino acid ester 4a in high yield with excellent enantiomeric and diastereomeric selectivity (Science 2018, 360, 1438). The study reveals that the whole catalysis can be characterized via three stages: (i) the catalyst 1a reacts with the tert-butyl glycinate 3 to generate the active carbanion complex IM3. (ii) IM3 then reacts with the N-diphenylphosphinyl imine 2a giving the imine intermediate IM8. (iii) IM8 undergoes hydrolysis to give the final product anti-4a and regenerate the catalyst 1a for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible for experimental realization. The hydrolysis step in the stage III is predicted to be the rate-determining step during the whole catalytic cycle. Furthermore, the origins of the enantioselectivity and diastereoselectivity for the target reaction, as well as the deactivation of the catalyst 1b, are also discussed.

Enantioselective Synthesis of Pyroglutamic Acid Esters from Glycinate via Carbonyl Catalysis

Direct α-functionalization of NH₂ -free glycinates with relatively weak electrophiles such as α,β-unsaturated esters still remains a big challenge in organic synthesis. With chiral pyridoxal 5 d as a carbonyl catalyst, direct asymmetric conjugated addition at the α-C of glycinate 1 a with α,β-unsaturated esters 2 has been successfully realized, to produce various chiral pyroglutamic acid esters 4 in 14-96 % yields with 81-97 % ee's after in situ lactamization. The trans and cis diastereomers can be obtained at the same time by chromatography and both of them can be easily converted into chiral 4-substituted pyrrolidin-2-ones such as Alzheimer's drug Rolipram (11) with the same absolute configuration via tert-butyl group removal and subsequent Barton decarboxylation.

Nickel-catalyzed enantioselective vinylation of aryl 2-azaallyl anions

A unique enantioselective nickel-catalyzed vinylation of 2-azaallyl anions is advanced for the first time. This method affords diverse vinyl aryl methyl amines with high enantioselectivities, which are frequently occurring scaffolds in natural products and medications. This C-H functionalization method can also be extended to the synthesis of enantioenriched 1,3-diamine derivatives by employing suitably elaborated vinyl bromides. Key to the success of this process is the identification of a Ni/chiraphos catalyst system and a less reducing 2-azaallyl anion, all of which favor an anionic vinylation route over a background radical reaction. A telescoped gram scale synthesis and a product derivatization study confirmed the scalability and synthetic potential of this method.

Organocatalyzed Cascade Aza-Michael/Aldol Reaction for Atroposelective Construction of 4-Naphthylquinoline-3-carbaldehydes

An organocatalyzed cascade aza-Michael/Aldol reaction of alkynals with N-(2-(1-naphthoyl)phenyl)benzenesulfonamides has been disclosed. In the presence of a secondary amine catalyst, this method enables the construction of a series of axially chiral 4-naphthylquinoline-3-carbaldehydes in yields of up to 97% with enantioselectivities of up to 96%. Several further transformations of the synthesized products were investigated to demonstrate their synthetic applications.

Direct Catalytic Asymmetric α-Allylic Alkylation of Aza-aryl Methylamines by Chiral-Aldehyde-Involved Ternary Catalysis System

A ternary catalytic system comprising a chiral aldehyde, a transition metal, and a Lewis acid is rationally designed for the asymmetric α-allylic alkylation reaction of aza-aryl methylamines and π-allylmetal electrophiles. Structural diversity chiral amines bearing carbon-carbon double bonds and aza-heterocycles are produced in moderate to good yields with good to excellent enantioselectivities. These products can be readily converted into other chiral amines without the loss of enantioselectivity. A reasonable reaction mechanism is proposed to illustrate the stereoselective control results.

Chiral Lewis acid-bonded picolinaldehyde enables enantiodivergent carbonyl catalysis in the Mannich/condensation reaction of glycine ester

A new strategy of asymmetric carbonyl catalysis via a chiral Lewis acid-bonded aldehyde has been developed for the direct Mannich/condensation cascade reaction of glycine ester with aromatic aldimines. The co-catalytic system of 2-picolinaldehyde and chiral YbIII-N,N'-dioxides was identified to be efficient under mild conditions, providing a series of trisubstituted imidazolidines in moderate to good yields with high diastereo- and enantioselectivities. Enantiodivergent synthesis was achieved via changing the sub-structures of the chiral ligands. The reaction could be carried out in a three-component version involving glycine ester, aldehydes, and anilines with equally good results. Based on control experiments, the X-ray crystal structure study and theoretical calculations, a possible dual-activation mechanism and stereo-control modes were provided to elucidate carbonyl catalysis and enantiodivergence.

A one-pot stepwise approach to axially chiral quinoline-3-carbaldehydes enabled by iminium–allenamine cascade catalysis

An organocatalytic atroposelective annulation between 2-(tosylamino)aryl ketones and 2-alkynals for the construction of enantioenriched axially chiral 4-arylquinoline-3-carbaldehydes is achieved.